This invention relates to an organic-electroluminescence (hereinafter often “organic-EL”) device, a process for its production, and an organic-electroluminescence display system having the device.
In general, in notebook personal computers, PDAs (personal digital assistants), mobile computers, portable information terminals, cellular phones and so forth, liquid-crystal display is chiefly used as flat-panel display. Also, in recent years, the proportion of using liquid-crystal display in place of CRT (cathode ray tube) display is increasing in desktop computers, too.
However, the liquid-crystal display has problems such that it has insufficient response speed, requires a large power consumption in the case of backlighting systems, has a low luminance and contrast in the case of reflection systems and has inferior visual angle characteristics.
Flat-panel display substitutive of such liquid-crystal display may include PDP (plasma display panel) and FED (field emission display). These, however, also has problems that they require a large power consumption, can not be made thin and are heavy-weight.
Accordingly, as display that can solve these problems in liquid-crystal display and other flat-panel display such as PDP and FED at a stretch, organic-EL display is proposed (e.g., C. W. Tang et al., Appl. Phys. Lett., 51, 913, 1987). This organic-EL display has various superior characters such that it has a very higher response speed than the liquid-crystal display, has an excellent viewing angle due to displaying by self light emission, can be made thin by half as much as the liquid-crystal display because it has only one sheet as a necessary glass substrate and hence can be light-weight and may require a smaller power consumption than the backlighting-type liquid-crystal display. Accordingly, the organic-EL display is expected as a prospective means in the twenty-first century.
The organic-EL display uses, as luminescent devices, organic-EL devices making use of organic compounds as luminescent materials. The organic-EL devices have basic structure in which the anode, a photoemission layer and the cathode are superposed in this order on a substrate. A hole transport layer between the anode and the photoemission layer, and an electron transport layer between the cathode and the photoemission layer are optionally provided. Color display by such organic-EL devices include full-color display by three-primary-color dot matrixes, and multiple-color area color display. In either case, photoemission layers must be formed in prescribed patterns.
As methods for forming the photoemission layers for color display, the following methods (1) to (4) are known in the art.
(1) A method in which respective EL luminescent low-molecular materials for red, blue and green are separately mask-vacuum-deposited three times;
(2) a method in which organic-EL blue-light emission is converted into red color and green color by means of color conversion layers;
(3) a method in which solutions of respective EL luminescent high-molecular materials for red, blue and green are coated by ink-jet printing to coat three-primary-color materials separately; and
(4) a method in which white-color EL light backlighting and color filters are used in combination.
However, the method (1) of mask vacuum deposition has so poor a productivity as to result in a high cost. Also, mask registration must be made inside a vacuum reactor, and it is difficult to achieve uniform film formation because of a difference in molecules' flying angle and distance between the middle area and the peripheral area of a substrate. In addition, there is a problem that any dusting inside the vacuum deposition reactor may cause film defects.
The method (2) of color conversion requires the color conversion layers other than organic-EL layers formed of EL luminescent high-molecular materials, and has a problem that many steps must be provided. In addition thereto, there is another problem that photoemission efficiency may lower because of a loss at the time of color conversion.
The method (3) of ink-jet coating requires dams for separating dots of EL luminescent high-molecular materials, and hence involves a low aperture percentage, resulting in a low effective luminance. Moreover, a long tact time is required because any whole-surface one-time coating can not be performed, resulting in a high cost especially in the case that large number of organic EL devices are produced in one lump substrate. There is also a problem on how to keep the quality of inks and printer heads.
The method (4) of using white EL light backlighting and color filters in combination has a disadvantage that the utilization efficiency of organic-EL light is so poor as to require a large power consumption.
Accordingly, as disclosed in Japanese Patent Application Laid-open No. 11-8069, a process for producing an organic-EL device making use of a photocurable acrylic resin is proposed. This process is a process in which a photosensitive resin composition prepared by adding an organic-EL material to a photocurable acrylic resin is coated on a substrate to form a film, followed by exposure via a mask having prescribed patterns and then development to form a photoemission layer and a hole transport layer and/or an electron transport layer in that prescribed patterns.
According to this process, the layers can be formed in patterns in a simple manner for each color of RGB (red, green and blue). The acrylic resin, however, may be affected by oxygen the atmosphere may contain at the time of curing, so that the surface may cure with difficulty. Especially in the production of organic-EL devices in which photoemission layers must be formed in thin films of about 100 nm thick, the rate of curing in air is so greatly low that the layers must be exposed in an inert atmosphere of argon, nitrogen or the like. This can be an obstacle to the achievement of mass production of devices. Also, since a liquid is used as the photosensitive material, a gap must be provided between the photosensitive material and the mask, and hence no precise exposure can be effected.